Although a range hood can achieve a function of discharging air into a room and discharging the air outside the room, the internal circulation air passage is too long when the air is discharged into the room, which causes a relatively large loss of air quantity and air pressure. While the range hood with double fans has a relatively large left-and-right size and a relatively high cost; since a switching plate is opened or closed towards an external direction of a volute casing spiral, the air pressure in a volute casing air passage will jack up a movable housing part, and in particular when the air pressure is large at a high speed level, a connection gap jacked up will generate an air flow whistle with high frequency, which results in poor sound quality.